Add async execution, structured diagnostics, and distinct platform regressions

[LunaStev]

This change adds lazy async functions and explicit await suspension, moves additional frontend and backend failures onto structured diagnostics, and replaces selected repeated platform cases with distinct workloads. Ordinary fun main() drives async work through std::task::block_on.

The implementation includes backend-neutral resumable frames, a cooperative task executor, readiness integration, a Windows IOCP provider, cancellation acknowledgement, IPv6 async socket helpers, and exact-length reads. Generic method rewriting and typed receiver handling accompany the frontend changes. Explicit MSVC target/linker handling is added while Windows GNU remains available.

Eight existing cases are strengthened and two shared cases are added. They cover bounded streaming search, syntax layouts, architecture-specific aggregate/argument transport, epoll pipe demultiplexing, kqueue-driven parsing, Windows allocation/counting sort, and a FreeBSD bit-graph workload. Earlier platform-matrix cleanup removes the unsubstantiated macOS/iOS/Windows RISC-V cells and records DragonFly amd64 as disabled/planned; it does not claim DragonFly execution support.

Validation and review status:

• Rust formatting has been applied.
• The final commit has not been compiled or tested locally, following the request to defer verification. Earlier intermediate results are not evidence for this final revision.
• Native IOCP execution, cancellation races, cross-target async execution, and the new cases still require validation. FreeBSD cases that use compile-only metadata remain compile-only.
• This PR is open for review. Known CI failures and remaining async/MSVC validation are tracked in the linked issues for follow-up work.

Closes #516
Closes #517
Closes #518
Closes #519

Related: #362, #395, #397, #398, #399, #409, #435, #437, #439, #440, #441, #442, #443, #444, #445, #292, #498, #515.

Follow-up audit:

Remote CI has now run at this head. Strict Clippy fails on three redundant closures in async lowering; case checking finds reserved byte identifiers in two new workloads. The LoongArch workload also exposes the existing arithmetic-index limitation (#436), and native Windows ARM64 still reports the previously tracked access violation (#493). The Windows GNU amd64 case job separately encountered an MSYS2 download timeout; that single transport failure is not being reported as a new compiler defect. These observations do not establish that later blocked steps pass.

24 focused follow-up issues were opened after comparing existing reports. Source-only findings are explicitly distinguished from observed remote CI failures in their bodies:

• Paths and duration: Prevent an out-of-bounds terminator write in path_dirname_copy #521, Account for nanosecond carry and borrow before rejecting Duration overflow #522, Distinguish Windows drive-relative, rooted and UNC paths in path_is_abs #529.
• Windows filesystem and environment: Preserve and classify native Windows filesystem errors #523, Return real handle metadata from Windows fstat #524, Use Unicode Windows APIs for UTF-8 filesystem paths #525, Convert the Windows environment block to UTF-8 instead of copying narrow API bytes #526, Honor append and truncate flags in native Windows file opening #527, Do not report every existing Windows path as accessible for every mode #528.
• Windows readiness and async lifetime/resource use: Prevent low-index sockets from starving Windows event_wait results #530, Keep live sockets usable across task executor shutdown and restart on Windows #531, Reclaim inactive async descriptor watch groups during long-running workloads #532, Avoid full task-list scans for async identity lookup and dependent wakeups #533, Bound IOCP staging allocations independently of caller buffer capacity #534, Surface invalid async sleep durations instead of discarding timer errors #535.
• MSVC artifact publication: Publish MSVC import libraries using final output names when staging linked artifacts #536.
• Case runner and workload coverage: Allow the Wave case runner to show help before locating a compiler #537, Stop classifying valid program stderr as a compiler failure by substring #538, Replace remaining generated platform copies with layer-specific functional workloads #542.
• JSON input validation: Reject unescaped control bytes in JSON strings #539, Bound JSON nesting before recursive metadata parsing exhausts the stack #540, Reject out-of-range JSON numbers instead of silently serializing them as null #541.
• Observed PR CI blockers: Restore strict Clippy CI after async lowering adds redundant closures #543, Fix reserved byte identifiers in the new streaming workload cases #544.

Good first issues: #521, #537, #539, #541, #543, #544.

These follow-ups remain open; they are not additional automatic-closing references.